Iron and iron-based metal-containing alloys, such as mild steel, are well-known materials used in constructing the apparatus of aqueous systems. In these systems water circulates, contacts the ferrous-based metal surface, and may be concentrated, such as by evaporation of a portion of the water from the system. Even though such metals are readily subject to corrosion in such environments, they are used over other metals due to their strength and availability.
It is known that various materials which are naturally or synthetically occurring in the aqueous systems, especially systems using water derived from natural resources such as seawater, rivers, lakes and the like, attack ferrous-based metals, the term "ferrous-based metals", as used herein, shall mean iron metal and metal alloys containing iron therein, i.e., ferrous metals. Typical devices in which the ferrous-metal parts are subject to corrosion include evaporators, single and multi-pass heat exchangers, cooling towers, and associated equipment and the like. As the system water passes through or over the device, a portion of the system water evaporates causing a concentration of the dissolved materials contained in the system. These materials approach and reach a concentration at which they may cause severe pitting and corrosion which eventually requires replacement of the metal parts. Various corrosion inhibitors have been previously used.
Chromates and inorganic phosphates or polyphosphates have been used in the past to inhibit the corrosion of metals which is experienced when the metals are brought into contact with water. The chromates, though effective, are highly toxic and, consequently, present handling and disposal problems. Phosphates are nontoxic. However, due to the limited solubility of calcium phosphate it is difficult to maintain adequate concentrations of phosphates in many instances. The polyphosphates are also relatively non-toxic, but tend to hydrolyze to form orthophosphate which in turn like phosphate itself can create scale and sludge problems in aqueous systems (e.g. by combining with calcium in the system to form calcium phosphate). Moreover, where there is concern over eutrophication of receiving waters, excess phosphate compounds can provide disposal problems as nutrient sources. Borates, nitrates, and nitrites have also been used for corrosion inhibition. These too can serve as nutrients in low concentrations, and/or represent potential health concerns at high concentrations.
In addition, environmental considerations have also recently increased concerns over the discharge of other metals such as zinc, which previously were considered acceptable for water treatment.
Much recent research has been concerned with the development of organic corrosion inhibitors which can reduce reliance on the traditional inorganic inhibitors. Among the organic inhibitors successfully employed are numerous organic phosphonates. These compounds may generally be used without detrimentally interfering with other conventional water treatment additives. These organic phosphonates, however, can be degraded to form orthophosphate, raising concern over eutrophication of receiving waters. This has led to restriction on the discharge of organic phosphonates by some regulatory bodies. Despite a number of reports of non-phosphorus organic corrosion inhibitors, no agent of this type has enjoyed wide commercial acceptance.
There is a continuing need, therefore, for safe and effective water treatment agents which can be used to control corrosion. In particular, there is a need for treatment agents that are based on non-phosphorus-containing organic corrosion inhibitors.